Modem digital telecommunication systems provide source coding of signals (for example, languages) with the aid of which compression of the signal is achieved such that a substantially lower bit rate than that of the (primary) digital signal suffices for transmission. Thus, in accordance with the system standard of the generally known GSM mobile radio system, the speech signal is sampled at the transmitter end at a rate of 8000 samples per second, the samples being represented with a resolution of 13 bits. This corresponds to a bit rate of 104 kbit/s per speech signal. The source or speech encoder compresses this speech signal to a source-coded speech signal with blocks of lengths 260 bits and a bit rate of 13 kbit/s. A compression of the speech signals by the factor 8, therefore, takes place.
The physical conditions in the case of wireless telecommunication, specifically in the frequency bands, available for mobile radio systems, under terrestrial conditions in the case of which there is multiple scattering and reflection on natural obstacles, lead to high and relatively strong fluctuating propagation losses and fading produced by multipath propagation (fast fading). In individual time sections (timeslots) of the transmission process, the transmission can thereby be strongly disturbed or even completely interrupted, while other timeslots are, by contrast, scarcely disturbed.
The useful datastream therefore, includes phases which either have a higher or lower bit error rate; that is, the errors occur in bursts, in particular.
Because of these circumstances, the transmission of the speech signal, which is highly compressed and reduced in redundancy by the source coding, will not be possible directly with acceptable quality. The bit error rate to be expected (of the order of magnitude of 10−3 to 10−1) is, therefore, to be reduced to acceptable values (of the order of magnitude of 10−3 to 10−6) by suitable error correction methods. This is the task of channel coding, which basically adds a (defined) redundancy once again to the source coded signals, which then permits the detection and correction of transmission errors on the transmission link (air interface).
In the method of the generic type, channel coding includes error protection coding (convolutional coding) and interleaving (also denoted as scrambling). Subsequently, the convolutionally coded and interleaved blocks are encrypted, mapped onto data bursts, modulated onto the carrier frequency, and transmitted.
It must be kept in mind for convolutionally coding that the source coded bits are not of equal relevance for the speech quality after decoding. Errors in some bits lead to substantial impairments of comprehensive validity, whereas errors and other bits are scarcely perceptible. The source coded bits are therefore split into pulses or groups, where each pulse or group is provided with a different error protection. Thus, in the case of GSM full-rate encodec (encoder/decoder for full rate transmission) they are the protection classes 1a, 1b, and 2.
A conventional method for implementing this different protection is what is termed “puncturation” or puncturing of the code following upon the error protection coding (convolutional code). Simply, the puncturation eliminates one or more positions from the output bit stream of the convolutional encoder in accordance with a prescribed scheme (a puncturation table). A puncturation table consists of elements 0 and 1, and is periodically processed, the bit corresponding to a 0 not being sent in the output bit stream, and the bit corresponding to a 1 being transmitted. The coded sequence is consequently shortened, and the error protection effect is weakened. Punctured codes have the advantage, however, of the implementability of various coding rates, codes with a higher coding rate being developed by periodic puncturation starting from a mother code of rate 1/n.
The fundamental sensitivity of the convolutional coding and decoding methods to errors occurring in bursts, such as those which constitute the main problem in the case of mobile radio connections, is still further sharpened by the puncturation such that punctured convolutional codes are applied virtually only in conjunction with subsequent nesting or interleaving. An approximately constant fading amplitude occurs within a timeslot in the case of typical mobile radio channels with the profiles TU (Typical Urban) or HT (Hilly Terrain) in the GSM system or in similarly profiled channels. If another channel is used for each new timeslot (ideal frequency hopping), the fading amplitude can be assumed to be statistically independent between two timeslots. Consideration as interleaving methods is given for this purpose to block interleaving or random interleaving, for example, which are known as such to the person skilled in the art.
The determination of effectively punctured convolutional codes likewise constitutes an important design task for a digital telecommunication transmission system of the type outlined above such as the selection and concrete configuration of the interleaving method to be applied in accordance with the error protection coding. Systematic instructive methods are not known for this purpose, and so it is necessary in order to find effectively punctured convolutional codes to conduct computer-aided testing of various codes and puncturings and compare them with one another with the aid of specific criteria which permit a statement on the likely transmission quality. It is typical, in this case, to presuppose that the interleaver is ideal (“infinitely deep”).
It is an object of the present invention, therefore, to specify a method for channel coding which is improved with regard to the achievable transmission quality.